Plants and predators pick same poison

Caterpillars and their hosts independently evolved identical recipe for cyanide

There’s a patent war pending over the invention of the cyanide bomb.

GRAZERS BEWARE Nibble on bird’s-foot trefoil, a plant in the pea family, and you’ll get a mouthful of poisonous cyanide, which dissuades most grazers. A caterpillar known to safely munch on the plant can make cyanide of its own, having independently evolved the same instructions and cellular machinery that the plant uses, scientists now show. Martin Olsson/Wikimedia Commons

PALATE FOR POISON Zygaena caterpillars, which deter hungry birds by secreting droplets of cyanide (circled), get the poison by eating poison-laced plants or, it turns out, can make it in-house. The plants and insects use identical recipes and machinery to make the poison, a new study reveals. N.B. Jensen et al/Nature Communications 2011

Zygaena caterpillars, which deter hungry birds by storing the poison in their flesh, make cyanide using the exact same cellular machinery as their host plants, scientists report April 12 in Nature Communications. It still isn’t clear which came up with the recipe first, but the discovery is the first known example of organisms from entirely different kingdoms evolving the same biochemical treachery.

Some plants, such as bird’s-foot trefoil, concoct cyanide bombs that are trip-wired to blow up in the mouths of nibbling animals. When a slug or insect chews a leaf, ingredients that are kept in different compartments in the plant’s cells combine to form cyanide, poisoning the animal.

Scientists knew that some caterpillars could safely eat cyanide-laced plants and store the poison in their bodies. But they recently discovered that when host plants are cyanide-poor, the caterpillars can make the poison themselves as a means of deterring their own predators.

“We could rear the caterpillars on plants that had none of these cyanogenic glucosides and the insects still produce the compound,” says study coauthor Birger Møller, of the University of Copenhagen. “We had no clue how they were making it.”

The researchers first speculated that sometime in the evolutionary past, the caterpillars had stolen the genes containing the instructions for making cyanide from the plants. But the sequence of DNA letters in the plants’ and caterpillars’ cyanide genes looked nothing alike, Møller and his colleagues discovered. Strangely though, the plants and caterpillars both use genes from the same superfamily that are found in pretty much all living things (humans use genes from this family to break down toxins in the liver).

Not only do both organisms record their cyanide-concocting instructions in three similar but very distantly related genes, they also build the poison with the same cookware. The enzymes working the molecular assembly line leading to cyanide are the same, modifying the same molecular ingredients in both creatures, the researchers report.

“They are using the exact same chemistry and enzymes. It really is not what we would have expected,” says David Gang of the Institute of Biological Chemistry at Washington State University in Pullman. “It is like inventing the wheel twice.”

Which came first, the plant or caterpillar version, is under investigation, says Møller. Both scenarios are plausible — if the plants figured it out first, the caterpillars could exploit a new ecological niche, once they figured out how to safely sequester the toxic compound in their body. Conversely, if the caterpillars were already making cyanide and they chanced upon plants that made it as well, the caterpillars could save energy and resources by getting the poison from the plant rather than building it themselves.

“If you ask an insect person, they think the insect did it first,” Møller says, “if you ask a plant person, they think the plants did it first.”

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